PEP-II NEW IR ISSUES

1. PEP-II NEW IR ISSUES M.Biagini LNF, INFN PEP-II MAC Break-out Session SLAC, Oct. 10th 2003

2. Luminosity Upgrade & IR issues • To increase Luminosity in PEP-II there are few key points (brute force): • Decrease by* • Decrease sz • Increase number of colliding bunches • Increase currents • Increase beams separation to decrease the effect of parasitic crossings • All these... leaving the present IR as much unchanged as possible !

3. Luminosity Upgrade & IR issues (cont’d) • These goals are problematic with the present IR: • Q1 is not strong enough to lower by* • Need to push Q1 closer to IP (gradient increases, balance between peak by in Q1 and chromaticity increase) • Parasitic crossings can be an issue (as now in by_2 pattern), degrading luminosity and tune shifts (separation is not enough) • At higher currents beam backgrounds can be a big problem (Sullivan)

4. Luminosity Upgrade & IR issues (cont’d) • At higher currents and shorter bunches HOM heating, beam pipe temperature and instabilities all grow • Decreasing sz thepeak current increases and also increases the probability of trapping modes (bunch spectrum is larger) • Chromaticity correction can be a problem: with present optics the b functions at the nearby sextupoles have been already increased in order to operate with present sextupoles • Vacuum pipe apertures can also be an issue when decreasing b* (peaks increase) and can limit beam lifetimes AND produce backgrounds • A smaller bunch length could affect the Touschek lifetime in LER (not an issue now) if there is not a corresponding increase in dynamic aperture

5. First solution • Introduce a small horizontal crossing angle: - increases beam separation - decreases effect of PC - allows for safe by_2 pattern collision • All operating Factories, except for PEP-II, have a small crossing angle: CESR ±2.3 mrad, KEK-B ±11 mrad, DAFNE ±12 to ±14.5 mrad. KEK-B has a reduction in L for bb tune shift > 0.05, and it is going to add crab cavities to get to higher luminosity • A preliminary design for PEP-II IR, with by* = 5 mm, has shown that ±3.5 mrad is “geometrically” easy to do. Need to remove 4 B1 slices (no need for further separation) and put 4 Q1 slices (need for more y-focusing gradient). The present correctors can match orbits to the present values.

7. Crossing angle issues • This solution has drawbacks: • Large angles can induce synchro-betatron resonances in the beams. Piwinski angle: Q = fsz/sx • Unwanted beam-beam interactions at Parasitic Crossings • Effect of off-axis trajectories in quadrupoles and solenoids on the beam optics have to be carefully evaluated • Luminosity and tune shifts are affected: L ¯, x ¯ (geometrical reduction) + reduction for high x at small angles (bb simulations)

8. Parasitic Crossings Effect The unwanted beam interaction at the PCs has 2 effects: • x and y tune shifts are induced, similarly to the main IP, depending on the beam separation at the PC • beam lifetime is affected, if the separation is lower than 10 sx x, y = beam separation at PCs Gaussian beam distribution J. Jowett, Handbook of Accelerator Physics and Engineering: Beam-beam tune shifts for gaussian beams

9. PEP-II PCs Tune shifts (June 2003) Y on log scale by_1 by_2 by_3 by_4 The PC tune shifts are normalised to the main IP tune shifts

10. Tune shifts due to PCs: new IR,c. a. Y on log scale New IR configuration, ±3.5 mrad crossing angle bx=28 cm, by=5 mm, xxLER=.098, xyLER=.076, xxHER=.076, xyHER=.062 Nb =1700, Npart/bunch=1.2x1011/6x1010 (+/-) (modified John’s numbers)

11. Luminosity & tune shifts with crossing angle For g >> tg (q/2). sz = bunch length, q = crossing angle P. Raimondi, M. Zobov, “Tune shift in beam-beam collisions with a crossing angle”, DAFNE Tech. Note G-58, 2003

12. Luminosity & tune shifts with crossing angle • These formulae are similar to the head-on collision ones if we simply substitute the horizontal beam size with: • A large crossing angle introduces strong coupling between the horizontal and longitudinal planes, provided that sz > sx (this is almost always true). • Luminosity and tune shifts are reduced with the crossing angle. However, since: the horizontal tune shift drops faster than the luminosity.

13. Luminosity with crossing angle Luminosity geometric reduction due to the crossing angle, vs c.a. Y scale: Luminosity with c.a. normalized to the head-on one. Lhead on = 3.3x1034

14. Tune shifts with crossing angle Tune shifts geometric reduction due to crossing angle, vs. crossing angle. Y scale: tune shift with c.a. normalized to the head-on one. Horizontal x drops faster. Beam footprint is smaller.

15. Cai’s beam-beam simulation Looks like small crossing angles are worst than large crossing angle!!

16. 2nd solution ? • Beam-beam simulations for PEP-II (Omhi, Cai) show that even a small crossing angle can degrade luminosity, so... Back to head-on collision... ? • To decrease by*, Q1 needs to be more powerful  add more slices. Fill the gap between B1 and Q1? (Sullivan, Ecklund) • To increase beam separation B1 field could be increased (SR increases too...) ? • The values of present correctors may not be large enough to match orbits to the present values at IR ends ( no perturbation is wanted outside IR) • The PC effect starts to be important

18. HER PC tune shifts in by_2 patternvs. by* and f Comparison of PC tune shifts for different by* and crossing angles. The head-on solution is the red curve

19. LER PC tune shifts in by_2 patternvs. by* and f Comparison of PC tune shifts for different by* and crossing angles. The head-on solution is the red curve

20. Conclusions • The present IR is very crowded, it is not simple to modify it without perturbations. But 2 solutions exist • Head-on solution with low-b shows huge effect of PCs • Crossing angle can still be a good solution if we can afford L degradation by pushing other parameters Need to study in more detail with bb simulations of PC the head-on vs crossing angle options !